`diag` for `BandedMatrix`es for off-limit bands (#56065)
Currently, one can only obtain the `diag` for a `BandedMatrix` (such as
a `Diagonal`) when the band index is bounded by the size of the matrix.
This PR relaxes this requirement to match the behavior for arrays, where
`diag` returns an empty vector for a large band index instead of
throwing an error.
```julia
julia> D = Diagonal(ones(4))
4×4 Diagonal{Float64, Vector{Float64}}:
1.0 ⋅ ⋅ ⋅
⋅ 1.0 ⋅ ⋅
⋅ ⋅ 1.0 ⋅
⋅ ⋅ ⋅ 1.0
julia> diag(D, 10)
Float64[]
julia> diag(Array(D), 10)
Float64[]
```
Something similar for `SymTridiagonal` is being done in
https://github.com/JuliaLang/julia/pull/56014
